It is well recognized that ambulatory blood pressure (BP) monitoring by means of wearable sensors has the potential to enable new levels of health-related vigilance and medical care in a number of novel settings, including, for example, controlling chronic hypertension and monitoring in-patients during convalescence. However, a significant challenge to realizing true non-invasive blood pressure (NIBP) measurement remains the problem of accounting for the unknown tension in the underlying arterial wall: If one simply measures pressure external to an artery (for instance, on the overlying skin), one is measuring the balance of intra-arterial pressure and the rapidly varying arterial wall tension. Ideal NIBP methods solve the problem of estimating intra-arterial wall pressures independently of the arterial wall tension. Yet, there is no optimal solution to truly wearable NIBP measurement. The ideal wearable device would be lightweight, easy-to-apply, non-invasive, small, unobtrusive, and as close to imperceptible as a regular wrist-watch or piece of jewelry. Existing NIBP measurement modalities, summarized as follows, have failed to meet the foregoing desiderata.
Oscillometric NIBP: The most common form of NIBP measurement, both in the home and in the clinical arena, uses the well-established oscillometric method. Oscillometry is based on the following principle: when the external cuff pressure equals the internal arterial pressure, the magnitude of arterial volume pulsations is maximal (because of the mechanical properties of arteries). While they are the standard-of-care, such devices nonetheless have shortcomings, including: (i) their circumferential compression of the extremity is uncomfortable, causing bruising or interrupting sleep; (ii) they are very susceptible to motion, since motion artifact cannot be distinguished from arterial volume pulsations, (iii) they are not readily miniaturizable so the device tends to be bulky, and (iv) overall, their accuracy versus a gold-standard is often inadequate. A more convenient version of the oscillometric cuff fits on the wrist, but measurement errors can arise if (v) the wrist is at a different vertical level from the heart, or (vi) if the patient has distal arterial fixed occlusions or vasospasm.
The Vasotrac Device and “Reverse Oscillometry”: MedWave's BP sensor places a small “balloon” over the radial artery, increases the external pressure applied to the underlying radial artery, and measures the volumetric pulsations in the “balloon.” It is therefore a variation of standard oscillometry, and the company describes the methods as “reverse oscillometry.” The device is unsuitable for ambulatory monitoring because of its size and power requirements, moreover, it is motion-sensitive, like other oscillometric methods.
The Volume Clamp Method: The Portapres®/Finometer® family of devices of Finapres Medical Systems enables non-invasive measurement of BP waveform from a finger using photoplethysmographic (PPG) technology. In numerous instances, published works may refer to this measurement modality as ‘photoplethysmography’, although such usage is imprecise, and it can leave the incorrect impression that the PPG and arterial blood pressure (ABP) signals are interchangeable. The device is attractive in that it offers a non-invasive, continuous BP digital artery measurement, and it offers an umbilical cord mechanism to correct for hydrostatic offset errors (relative to the heart). Yet it requires a bulky, power-thirsty actuation system to drive the pneumatic cuff. Moreover, it circumferentially constricts the finger and prevents venous flow. Within minutes, this can become very uncomfortable, so much so that the Portapres comes equipped with two different finger cuffs so that it can alternate its operation between two fingers to minimize this discomfort.
The Finapres uses the volume clamp method of Penaz, which is based on the following insight: if a PPG signal is not changing, neither is the arterial transmural pressure, and visa-versa. Using an extremely rapid servo system with a finger cuff actuator, the Finapres adjusts the pressure in a finger cuff to keep a reference PPG signal flat throughout systole and diastole; the method is thus known as the ‘volume-clamp’ since the finger's blood volume is held constant. The waveform of whatever cuff pressure is necessary to keep the PPG signal flat must be equal and opposite to the digital arterial ABP.
Pulse Wave Velocity: Pulse wave velocity (PWV), the speed at which a pulse is transmitted through the arterial tree, is a function of arterial blood pressure, and there has been extensive academic and commercial effort to use this as a monitoring modality. The modality is described, for example, by Young et al, J Clin Monitoring 1995. However, according to Chen, Med Biol Eng. Comput. (2000), “No one has succeeded in realizing a reliable blood pressure monitor when only the pulse wave velocity or pulse arrival time is used.” There are several reasons for these difficulties:                In addition to systolic BP and diastolic BP, PWV is also a function of the subject's vascular physiologic state.        Furthermore, PWV is also a function of the frequency content of their pulse waveform.        Raising and lowering the arms changes the hydrostatic pressure within the extremity, so PWV may be unreliable in any position except supine.        In a canine study, PWV was found to remain constant for all mean BP's below 100 mmHg. So PWV may not correlate with BP for individuals close to, or less than, the normotensive range (e.g. it may be difficult to detect over-medication, or monitor heart failure patients, with PWV).        PWV is distance divided by pulse transit time, but it is difficult to measure the distance accurately, or keep the distance constant.        To measure pulse transit time, a proximal and distal measurement are necessary. But the electrocardiogram (ECG) is a problematic marker of pulse onset, perhaps in part because of variable electromechanical delays. The fact that PPG measurements require the use of two distinct PPG sensors has also proven problematic.        
Arterial tonometer solutions: Tonometers are very difficult to use, even in controlled laboratory conditions, and thus they have no role in ambulatory blood pressure measurement. Applanation tonometry requires that the artery wall segment be perfectly flat and that the pressure sensor overlies only the flattened portion of the artery and not extend over the curved portion of the artery. In this configuration, the flat segment of the arterial wall is unable to affect the balance between the intra-arterial pressure and the external tonometer pressure. These conditions are so difficult to achieve in practice that the one commercially available tonometer comes with a separate oscillometric cuff to calibrate the unreliable (in terms of absolute arterial pressure) tonometer unit.
Volume plethysmographs, e.g. Empirical Technologies wrist sensor: These devices can be light-weight and low powered. However, a plethysmograph, which is a volume signal, is quite different from the pressure signal. The volume of pulsation is a complex function of the arterial pressure, the mechanical properties of the artery, and the pressure applied by the sensor.
Wireless implanted devices e.g. CardioMEMS Enclosure: Technology now exists to implant an arterial pressure sensor for long-term pressure monitoring. However, in most instances a non-invasive device is preferable, and much more attractive to the broad population, than an implanted option.